1. Field of the Invention
The invention relates to the field of radio frequency identification circuits and methodologies.
2. Description of the Prior Art
The antenna of an RFID reader used in transmitting a carrier signal to an RFID tag is operated in a tuned fashion. The antenna, or coil for low frequency inductively coupled systems, is typically operated in such a fashion that the voltage on the coil (and subsequently its magnetic field) is very uniform. The uniformity is important because any fluctuation in the transmitting coil when used as a receiving coil would be detected as a signal from a tag. Even when using a separate receiving coil, it is coupled with the transmitting coil such that slight deviations by the transmitting coil cause amplitude modulation or ON/OFF keying in the receiving coil which are again sensed by the receiving coil as a signal.
With more state-of-the-art RFID systems, the reader communicates with the tag by altering the amplitude of its transmitted signal. In other words, the transmitting coil is changed from a high voltage to a lower voltage and in most cases is usually turned completely off in rapid succession. With a large voltage change to the tuned transmitted coil the recovery time is quite long. Unfortunately, it is typically during this recovery period that a backscattered (or mutually inducted for low frequency H field operation) signal from a tag is sensed. It is important that the change in the voltage on the sensing coil in the reader of RFID tags be kept to a minimum so that the subsequent recovery be minimized.